The present disclosure relates to sheet discharge devices and image forming apparatuses and particularly relates to a technique for preventing dropping of paper sheets piled on a piling section.
Image forming apparatuses and the like include those equipped with a sheet discharge device configured to discharge paper sheets after image formation or after post-processing, such as punching or stapling, and receive and pile the discharged paper sheets on a sheet output tray. If in such a sheet discharge device, for example, a pile of paper sheets on the sheet output tray becomes significantly out of alignment during piling up of the paper sheets, subsequent paper sheets can no longer properly be piled on the sheet output tray. Therefore, a technique is employed in which when a detection sensor or the like detects that the top surface of a pile of paper sheets on the sheet output tray is not at a proper vertical position with respect to a sheet output port, the sheet output tray is lowered a certain amount and then lifted to correct the top surface of the pile of paper sheets on the sheet output tray to the proper vertical position.
Generally, the sheet output tray has an upward inclination in a direction of discharge of paper sheets. Therefore, paper sheets are piled on top of one another on the sheet output tray while taking an inclined position where their trailing ends in the direction of discharge are located below their leading ends in the direction of discharge. Thus, even if a large number of paper sheets are piled on the sheet output tray, the pile of paper sheets is less likely to collapse. Furthermore, because of the presence of the inclination, the paper sheet discharged through the output port in the apparatus body toward the sheet output tray moves on the sheet output tray while going down toward the apparatus body, which prevents dropping of the paper sheet from on the sheet output tray.
However, as stapled sets of paper sheets are successively piled on top of one another on the sheet output tray, their trailing end-side stapled portions of greater thickness are superimposed one on another and the thickness of the pile of sets of paper sheets is increased more at its trailing end in the direction of discharge of the sets of paper sheets than at its leading end in the direction of discharge, resulting in gradually reduced inclination of the top surface of the pile of sets of paper sheets. As a result, subsequently discharged paper sheets may slide off the sheet output tray or the pile of sets of paper sheets may collapse.
To solve the above problem, a technique is proposed in which when, during discharge of stapled sets of paper sheets to the inclined sheet output tray as in the above case, the superposition of stapled portions located at the same position in a pile of the sets of paper sheets on the sheet output tray reaches a predetermined amount, a subsequent stapled set of paper sheets is discharged to a location offset from that of the previous stapled set of paper sheets to avoid further superposition of the stapled portions and in turn avoid the thickness of the pile of sets of paper sheets from increasing more at its trailing end.
A technique is also proposed in which paper sheets are collected face down and face up alternately every predetermined number of sets and each set of collected paper sheets is stapled at a predetermined portion thereof and then discharged to an inclined sheet output tray as in the above technique to avoid superposition of the stapled portions and in turn avoid the thickness of a pile of sets of paper sheets on the sheet output tray from increasing more at its trailing end.